Few studies have examined the impact of mentoring (developing a special relationship with a nonparental adult) on educational achievement and attainment in the general population. In addition, prior research has yet to clarify the extent to which mentoring relationships reduce inequality by enabling disadvantaged youths to compensate for the lack of social resources or to promote inequality by serving as a complementary resource for advantaged youths. The results of a nationally representative sample of youths show (1) a powerful net influence of mentors on the educational success of youths and (2) how social background and parental, peer, and personal resources condition the formation and effectiveness of mentoring relationships. The findings uncover an interesting paradox--that informal mentors may simultaneously represent compensatory and complementary resources. Youths with many resources are more likely than are other young people to have mentors, but those with few resources are likely to benefit more from having a mentor--particularly a teacher mentor--in their lives. (Contains 2 figures, 5 tables and 6 notes.)